1. Field of the Invention
This present application relates generally to AuPt heteroaggregate dendritic nanoparticles and, AuPt alloy nanoparticles and methods of using the same as catalysts.
2. Description of the Background
Hydrogen fuel cells generate electricity from the reaction of hydrogen at the anode side, and oxygen at the cathode side of the cell. FIG. 1 illustrates the general construction of a conventional hydrogen fuel cell with the reaction used to produce electricity.
A particularly advantageous type of hydrogen fuel cell is the Proton Exchange Membrane (PEM) fuel cell. FIG. 2 illustrates a conventional PEM fuel cell.
Proton Exchange Membrane (PEM) fuel cell devices transform the chemical energy liberated during the electrochemical reaction of hydrogen and oxygen to electrical energy. A stream of hydrogen is delivered to the anode side of the membrane-electrode assembly. At the anode, hydrogen is catalytically split into protons and electrons. This oxidation half cell reaction is represented by:H22H++2e−
The newly formed protons permeate through a polymer electrolyte membrane to the cathode. The electrons travel along an external load circuit to the cathode side, thus creating a current output of the fuel cell.
At the same time, a stream of oxygen is delivered to the cathode side of the membrane electrode assembly, where oxygen atoms react with protons permeating through the polymer electrolyte assembly and the electrons arriving through the external circuit to form water molecules. This reduction half cell reaction is represented by:4H++4e−+O22H2OPure platinum (Pt) has been used as a catalyst to split hydrogen atoms at the cathode site. Unfortunately, Pt is easily poisoned by carbon monoxide such that amounts of carbon monoxide at a level of 10 ppm can be problematic. Presently, the primary industrial source of hydrogen is from reformed hydrocarbon fuels, such as methane, which yields carbon monoxide contaminated feeds. In fact, pure Pt is poisoned by carbon monoxide to such an extent that H2 oxidation does not occur below 170° C. when carbon monoxide is present at the level of about 1,000-2,000 ppm. This is problematic inasmuch as PEM fuel cell devices using Pt anodes must operate at temperatures below 100° C., the boiling point of water.
Widespread use of hydrogen in fuel cell devices in the near term will require H2 supplies generated from carbon-based sources (i.e. methane reforming and Water Gas Shift reactions). In the absence of a breakthrough in photochemical water splitting or some other transformation technology for producing water, fossil fuel and hydrocarbons are the only economically and technologically viable sources of hydrogen. Unfortunately, hydrogen produced by these methods is contaminated with carbon monoxide (CO) at levels that range from 100 ppm to 10,000 ppm. Commercial fuel cells that run on hydrogen fuels currently use Pt or Pt—Ru catalysts to activate the H2 in the anode chamber. Pt is required because it is the only metal that can activate hydrogen below the 100° C. temperature limit of a water-based PEM fuel cell. However, the presence of CO in low concentrations severely poisons the PT catalyst and drives the activation (light-off) temperature above 150° C. This shift in light off temperature renders the fuel cell inoperative because the water-based fuel cell membrane is destroyed above 100° C. Because there is no viable solution to this problem, the major PEM fuel cell manufacturers (Ballard, GM, Plug Power) have specified that only pure hydrogen could be used in their devices. Pure, CO-free hydrogen can only be prepared from electrolysis of water or arduous gas purifications steps. Both methods are remarkably expensive, energetically uphill and not easily amendable to large scale production. Large-scale implementation of hydrogen fuel cells will, thus, require breakthroughs in either the production of pure hydrogen feeds or the development in CO-tolerant anode catalysts.
Recent reports of a Au—Pt bimetallic system have indicated CO-oxidation and CH3OH electo-oxidation activities. For example, it demonstrated by Chandler et al. and Crook et al. that Au—Pt bimetallics have unusual activities with regard to CO oxidation. The synthetic methodologies used in these studies involve the use of organic dendrimers that coordinate the precursor complexes prior to reduction to the metallic state. The dendrimer method permits the formation of the very small, homogeneous and monodispersed particles with well defined compositions, but dendrimers are prohibitively expensive and furthermore the resulting particles are difficult to characterize due to their sub-nanometer size. Because of the thermodynamic immiscibility of Au and Pt throughout most of the composition and temperature range, alloy formation is rarely described and particles containing both elements are typically just referred to as “bimetallics” without reference to any specific architecture. The synthesis and characterization of Pt@Au and Au@Pt core shell particles have also been reported but their properties were not well defined and their architectures uncharacterized.
Thus, a need exists for an anode catalyst for hydrogen fuel cell devices which can better tolerate carbon monoxide, and continue to function at acceptable temperatures, i.e., at 100° C. or less.